1. Field of the Invention
The present invention relates to a purification process for methyl methacrylate (hereinafter may be abbreviated as "MMA") and also to a preparation process for MMA.
2. Description of the Prior Art
MMA is produced by esterifying methacrylic acid (hereinafter may be abbreviated as "MAA") or methacrylamide with methanol in the presence of sulfuric acid, a strongly acidic ion-exchange resin or the like.
Crude MMA which is obtained from this esterification process contains, in addition to water formed by the reaction and unreacted methanol and MAA, small amounts of lower boiling-point impurities, such as methyl acrylate (hereinafter may be abbreviated as "MA") and methyl propionate (hereinafter may be abbreviated as "MP"), derived from impurities in the raw materials.
It is however difficult to recover MMA from crude MMA by conventional distillation procedures because MMA is close in boiling point to these impurities or MMA forms an azeotrope with them.
As a method for separating MMA from crude MMA, it has been proposed, for example, (1) to remove methanol from crude MMA by the azeotropic distillation which uses a hydrocarbon (Japanese Patent Laid-Open Nos. 9740/1982, 180457/1983 and 203940/1983), (2) to remove methanol from crude MMA by the extractive distillation in which water is used as a solvent (Japanese Patent Laid-Open No. 24812/1979), or (3) to extract and separate MMA by using water and a hydrocarbon as extracting solvents (Japanese Patent Laid-Open No. 24814/1979).
With the development of optoelectronics in recent years, methacrylic resins are now increasingly used in a large volume in the above field of art. There is hence an increasing demand for the quality of high purity from the standpoint of optical performance, to say nothing of the standpoint of mechanical and chemical performance. However, the conventional preparation processes of MMA are not fully satisfactory not only from the process aspect but also from the economical aspect Moreover, impurities which would cause problems for the preparation of high-purity MMA cannot be fully removed even by these processes. Valuable components are usually recovered and recycled for reuse from the economical standpoint, especially in industrial processes. Impurities therefore gradually build up in the processes and may cause a quality reduction of the products unless their removal is perfect, even if they are contained in minute quantities in the raw materials.